The benefits of trench-less digging for installing underground utility lines are well known. A number of different types of devices are available for the purpose of installing underground utility services without cutting an open trench or otherwise disturbing the earths surface or items rest thereupon. These devices include percussion boring tools, rotary boring tools or earth augers, and push rod boring systems. The present invention relates to a rotary push rod boring system.
Each of these different types of underground boring devices has a specific purpose and specific operating characteristics. Their use depends on the type of soil in which the borehole will be formed, the length and diameter of the borehole, conditions at the job site, and a number of other factors.
One type of devices is a push rod boring system which is a relatively simple, compact device for sequentially thrusting an increasing length drill string composed of push rods through the ground from a small subsurface launching pit or surface location to reach a target pit or another surface location. A boring head at the leading end of the drill string forms a small initial borehole. Taking advantage of the greater stability provided by pulling a slender rod, rather than pushing, apparatus is subsequently pulled back with the drill string of push rods. For instance, an enlarged head member or utility lines can be attached to the leading end of the drill string and then be retracted forcibly back through the initial borehole.
Improvements to the above boring devices, and more specifically to push rod boring systems, include the development of tracking and monitoring devices for the progress and orientation of the boring head. Also, there have been various means developed to correct or change the path of the boring head as the tool progresses, for implementing a change in direction, or if the boring head begins to deviate from the desired path because of changing soil conditions, rocks, or other obstructions.
One method of directing the path of the drill string is to use a boring head which has a beveled leading edge which cause the head to deflect. Use of a simple pushing action then results in a continuous curved borehole. However, substantially straight boreholes can be formed by the simultaneous rotational and axial advance of the drill string in a progressing helically spiraling movement to form a substantially straight borehole through the soil.
When a directional change is desired, rotation is discontinued when the boring head is orientated along the new path. As the drill string continues to advance through the soil without rotation, the resultant soil forces on the boring head act to deviate from its straightline course and to move instead along a curved path. As long as the bevel face of the boring head is first maintained in this same orientation, the path of the drill string will follow a continuous curve.
After the steering correction or change is completed, combined axial and rotational movement is continued.
Directional monitoring means include attachment of an electronic transmitter, sonde or similar device to the leading end of the drill string, usually combined with a boring head having a cutting tip or bevel face. Electronic transmitters, sondes or similar devices are used to monitor progress of the boring head, and determine the need for path corrections or changes, and to indicate the orientation of the bevel face.
More specifically, in U.S. Pat. No. 4,306,626 to Duke (Duke '626), a hydraulic push rod boring system is disclosed comprising a basic push rod boring system without directional control. This push rod system incrementally advances push rods into a bore by gripping the rod with a jaw mechanism that is thrust forward by a hydraulic cylinder. This form of mechanism is similar to that used with hand-held caulking guns. At the end of each cylinder stroke, the jaws are released from the rod and the cylinder is retracted for the next pushing increment. Additional push rods are added to the back end of the drill string as needed.
The addition of beveled face boring head would enable Duke's apparatus to form a deviated borehole, however, without the further addition of means for imparting rotary motion to the drill string, Duke cannot directionally control the drilling.
Thus, when a directional-boring head is used with this push rod system, the drill string of push rods must be rotated manually by the crew through the use of pipe wrench by pushing on the jaw handle, a tedious and inefficient method.
An automated rotation system is disclosed in U.S. Pat. No. 4,694,913 to MacDonald et al., discloses a rotary push rod boring system with bevel face boring head, having directional control. In order to achieve rotational motion necessary for directional control, this device uses an independent motor and control assembly, which provides either axial movement or combined axial and rotational movement to the drill string. This device requires however, a complex and expensive control mechanism.
As an improvement to the apparatus of Duke, U.S. Pat. Nos. 4,945,999 to Malzahn (Malzahn '999) and 5,070,948 to Malzahn et al. (Malzhan '948) disclose the addition of directional control and steering capabilities to the basic hydraulic push rod boring system. Malzahn '999 and '948 attach conversion devices to a push rod system so as to provide both axial and rotational movement of the drill string without using independent motors or other power sources, in other words, use the push stroke to effect rotation as well.
The conversion devices of Malzahn '999 and '948 comprise, in a first instance, a rigid link mounted between a fixed point and the jaw of Duke '626. In second instance a conversion device comprises, a cam follower mounted on the jaw and a rigid cam guide extending diagonally across the apparatus, and in a third instance, the jaw is pinned between intermediate cables, the cables ending at a leading fixed point positioned to one side of the jaw and at a rearward fixed point positioned to the other side of the jaw. The result of all of these conversion devices is to cause the jaw to be rotated a total of about 60 degrees.
Actual field use of these hydraulic rotating push rod systems has shown that a number of improvements are needed to increase the effectiveness, and overcome a number of existing known problems.
The plate type gripping assembly of Duke and as disclosed in Malzahn '999 and '948 damages the exterior surface of the push rods during each push stroke. The locking action of the opposing single plane movement of the gripping assembly plates, creates transverse or crosswise marks, scoring, indentations, and can even bend the push rod out of true alignment during high load push and operations. Misalignment of the locking plates on areas of previously damaged push rods adversely affects positive gripping and subsequent advance or retraction of the drill string.
Further, the plate-type apparatus results in slow forward movement, with a large number of strokes required to complete a borehole. Each forward stroke of the hydraulic thrust cylinder pushes the drill string forward only about nine inches and a maximum of 60 degrees of clockwise rotation. Thus a typical four-foot long push rod is pushed its length and rotated only 320 degrees clockwise, through just over five complete (forward and back) strokes of the hydraulic thrust cylinder.
The conversion devices Malzahn '999 and '948 introduce lull periods in the rotation, specifically as the jaw passes through the middle portion of the stroke. At this middle point, the path defined by the links, cables and tracks are substantially tangent to the push rod movement and thus impart very little rotation. In the straight drilling portion of a borehole, the rotational lull coupled with the small overall slow rotation does not create a tight helical spiral, nor one that is particularly straight. In fact, the forward progression of the boring head, and the subsequent borehole, tend to form an asymmetric helical spiral through the soil.
Movement through the distorted helical borehole in either direction may experience areas of binding. As a result, and exacerbated because the drill string is composed of individual push rods threaded together end-to-end, there is an increased likelihood of rod bending, and thread damage to push rod joints.
Tracking and orientation of the bevel face of the boring head is also affected by the slow rotation, lag and uneven rotation of the drill string. During drilling, consistent high and continual rotation of the drill string is required for effective directional control and monitoring of the orientation of boring head.
The conversion devices are exposed and the potential for damage during use is increased. Further, under the repetitive cycling, cables in the conversion devices experience wear and stretch, which require constant monitoring and adjustment.
Additionally, as stated above, following completion of a borehole, the drill string is pulled back with a bore enlarger or lines of some sort. This requires significant pulling force, greater than the force used to push the rods. Applicant notes that the hydraulic cylinders used in the known designs are of the conventional design and thus employ the return stroke of the hydraulic cylinder which, having reduced fluid area due to the piston rod, results in reduced pulling capacity. This is contrary to the need for increased capacity to overcome the substantial increases in resistance encountered during pull-back of drill strings with attached apparatus.
Existing hydraulic push rod systems employ a single coaxial hydraulic cylinder. No provisions are known for quickly and simply increasing or decreasing the capacity of hydraulic push rod boring systems with the coupling or removal of additional hydraulic cylinders as needed.
The difficulties and problems suggested in the preceding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness of these systems. Other noteworthy problems may also exist; however, those presented above demonstrate further improvements are needed to known hydraulic rotating push rod systems.